Extract Document preview without opening SolidEdge

in our company we have a small exe which extracts from every SE document a preview picture (200 X 200) without opening Edge.

I would like to rewrite this small tool for a project of mine (don't want to call the exe). Anyway, I've looked into the assembly SEPreviewLib

but couldn't figure out a way to extract a picture. I don't want to use Thumbnails. The only way to load a document is afaik with the SolidEdgeFileProperties assembly. Trying to cast the application object from SolidEdgeFileProperties into the Document interface failed... dang!